Ari Kankkunen

Vapor pressure and boiling point elevation of black liquor

The objective of this paper is to test Raoult’s law for calculating the boiling point elevation and the vapor pressure of black liquor. The vapor pressure of black liquor is an essential variable in black liquor evaporation, spraying and black liquor droplet drying during combustion. Typically only the boiling point elevation at the atmospheric pressure is reported and no simple models to estimate the vapor pressure are available. The model presented here is based on Raoult’s law for a binary solution of water and dry solids matter. Starting from this we derived an equation for boiling point elevation as a function of pressure and dry solids content. This model was then fitted to experimental data on boiling point elevation available for 14 different black liquors. The only free variable for single black liquor is the apparent molar mass of dissolved species. The model presented gave a good correlation with experiments, we tested 14 black liquors and the accuracy was better than 0.5°C at the range applicable for spraying conditions.